With the recent rapid increase in the amount of information communicated, reductions in size and weight and increases in the speed of information communication devices, such as PHS and cellular phones, are strongly demanded, and electrical insulating materials with low dielectric constants that can cope with such demands are required. In particular, portable mobile communications such as automobile phones and digital cellular phones, mobile communication devices such as satellite communications, and like devices use radio waves at high frequencies in the MHz to GHz bands. Further, due to the decrease of usable wavelength bands, high-frequency bands such as microwave and milliwave bands are increasingly used. Furthermore, the CPU clock time of computers has reached a GHz band, and higher and higher frequencies are used. To reduce the size and weight of communication devices operating at such high-frequency bands, it is necessary to develop an electric insulating material that has both excellent RF transmission characteristics and low dielectric properties.
In many cases, circuit board materials of electronic devices are required to have low dielectric properties, such as a low dielectric constant, a low dielectric loss tangent, etc., and excellent physical properties, such as high heat resistance, high mechanical strength, etc. The dielectric constant (∈) is a parameter indicating the degree of polarization in a dielectric, and the higher the dielectric constant, the greater the propagation delay of electrical signals. Therefore, to increase the propagation velocity of signals and enable high-speed operation, a low dielectric constant is preferable. The dielectric loss tangent (tan δ) is a parameter indicating the amount of a signal lost by conversion to heat during propagation through a dielectric, and the lower the dielectric loss tangent, the smaller the signal loss and the higher the signal transmission rate.
That is, energy loss in a transmission process, which is called dielectric loss, is caused in electronic circuits, and is not preferable since the lost energy is released in the electronic circuits as thermal energy. In low-frequency bands, such energy loss is caused because dipoles generated by dielectric polarization oscillate due to the change of the electric field, and in high-frequency bands, it is caused by ionic polarization or electronic polarization. The ratio of the energy consumed in a dielectric per cycle of an alternating electric field to the energy stored in the dielectric is called a dielectric loss tangent and expressed as tan δ.
In high-frequency bands, tan δ increases with an increase in frequency, and high-density packaging of electronic devices increases the amount of heat generated per unit area. Therefore, a material with a low tan δ needs to be used to achieve low dielectric loss in an insulating material. The use of a low-dielectric polymeric material with low dielectric loss suppresses heat generation by dielectric loss and electrical resistance, thereby reducing signal malfunctions. Thus, there are strong demands for materials with low transmission loss (energy loss) in the field of high-frequency communications.
Materials having electrical properties such as electrical insulation, low dielectric constant, etc., include polyolefines, vinyl chloride resins, fluororesins, and like thermoplastic resins; unsaturated polyester resins, polyimide resins, epoxy resins, bismaleimide triazine resins (BT resins), crosslinkable polyphenylene oxides, curable polyphenylene ethers, and like thermosetting resins; etc. Various types of such resins have been developed to satisfy the following properties:                drilling processability and cutting processability of laminated plates;        high heat resistance;        low coefficient of linear expansion;        adhesion or bonding to metal conductor layers (copper foil adhesion);        mechanical strength;        thin-film-forming ability;        a dielectric constant that can be selected as desired from a relatively wide range;        insulating properties;        weather resistance; and        low dependency of dielectric properties on temperature and humidity.        
However, resins as mentioned above have the following problems.
(1) Polyolefins
Polyolefins, such as polyethylenes and polypropylenes, have the drawback of low heat resistance, although they have excellent electrical properties such as high insulation resistance, since they have C—C bonds or like covalent bonds and contain no highly polar groups. Therefore, they exhibit impaired electrical properties (dielectric loss, dielectric constant, etc.) at high temperatures, and thus are not suitable for insulating films (layers) for capacitors and the like.
Polyethylenes and polypropylenes are made into films and then bonded over electrically conductive materials with adhesives. Such a process not only involves complicated steps but also has problems in film formation, such as extreme difficulties in forming thin films.
(2) Vinyl Chloride Resins
Vinyl chloride resins have low heat resistance like polyolefins, and have high dielectric loss, although they exhibit high insulation resistance, excellent chemical resistance and excellent flame retardancy.
(3) Polyvinylidene Fluorides, Trifluoroethylene Resins and Perfluoroethylene Resins
Although these polymers, which contain fluorine atoms in their molecular chains, have excellent electrical properties (low dielectric constant and low dielectric loss), high heat resistance and high chemical stability, they have drawbacks in molding processability and film-forming ability, such that they need to be heat-treated, like thermoplastic resins, to obtain molded articles, films, etc. Thus, considerably high cost is required to fabricate devices from such resins. Further, since they have low transparency, they have the additional disadvantage of offering limited applications.
(4) Epoxy Resins
Epoxy resins satisfy the requirements for insulation resistance, dielectric breakdown strength and heat resistance, but they have a relatively high dielectric constant of not less than 3, failing to satisfy the property requirements, and also have the drawback of poor thin-film-forming ability. A curable modified PPO resin composition is known, which is obtained by mixing a polyphenylene oxide (PPO) resin, polyfunctional cyanate resin and other resins, followed by the addition of a radical polymerization initiator and a preliminary reaction. However, the resin composition does not have a satisfactorily low dielectric constant. Further, to improve the poor heat resistance of epoxy resins, the combined use of epoxy resins with, for example, phenol novolac resins, vinyl triazine resins, etc., is being studied, but such combined use is disadvantageous in that the resulting film has extremely poor mechanical properties. To solve the above problems, i.e., to improve heat-processability, and adhesion and bonding to metal conductors (layers) such as copper, while maintaining electrical properties, branched cyclic amorphous fluoropolymers, copolymers of perfluoroethylene monomers and other monomers, etc., have been proposed. Such polymers and copolymers have satisfactory electrical properties such as dielectric constant and dielectric loss, but they have poor heat resistance due to methylene chains present in their polymer chains, and do not satisfactorily adhere to device substrates and the like.
(5) Polyimides, Polyethersulfones, Polyphenylene Sulfides, Polysulfones, Thermosetting Polyphenylene Ethers (PPEs), Polyethylene Terephthalates
Since device fabrication processes always include a soldering step, low-dielectric-constant materials with excellent dielectric properties and insulation resistance are further required to have sufficient heat resistance to withstand heating at least at 260° C. for 120 seconds, excellent chemical stability such as high alkali resistance and the like, moisture resistance and mechanical properties. Polymeric materials satisfying these requirements are known and include polyimides, polyethersulfones, polyphenylene sulfides, polysulfones, thermosetting polyphenylene ethers (PPEs), polyethylene terephthalate, etc. However, even these resins have high dielectric loss in the GHz band.
Thus, various difficulties are encountered in achieving the above properties by using only resins, and therefore the addition of additives to resins has been proposed for improving the electrical properties of resins. For example, Japanese Unexamined Patent Publication No. 1996-134263 discloses that addition of a certain amount of a specific metal silicate-based fibrous material to a synthetic resin improves the thermal conductivity, heat resistance and mechanical strength without increasing the dielectric constant and dielectric loss tangent to such an extent as to hinder the use in high frequency ranges, and in some types of resins, remarkably decreases the dielectric loss tangent while maintaining the same degree of dielectric constant, and thus resins containing such fibrous material can be used extremely advantageously as circuit board materials, and in particular circuit board materials for high-frequency applications, which are different from the conventional electrical applications of electrical and electronic components to which resins are applied.
More specifically, Japanese Unexamined Patent Publication No. 1996-134263 proposes a resin composition for high-frequency electronic components obtained by adding, to a thermoplastic resin (excluding polyamide resins) and/or a thermosetting resin (excluding phenol resins), reinforcing fibers comprising as a main ingredient a metal silicate-based fibrous material represented by the formula aMx.Oy.bSiO2.ocH2O (wherein a, b and c are each a positive real number; when x is 1, y is 1; when x is 2, y is 1 or 3; and M is at least one metal element selected from the group consisting of Mg, Cr, Mn, Fe, Co, Ni, Cu, Zn, Al, Ga, Sr, Y, Zr, Nb, Mo, Pb, Ba, W and Li), the amount of the reinforcing fibers being 5 to 60 wt. % based on the total weight of the resin and fibrous material.
In Japanese Unexamined Patent Publication 1996-134263, the reinforcing fiber is added to a thermoplastic resin or thermosetting resin in an amount of at least about 5 wt. %, indicating that the reinforcing fiber needs to be used in a large amount.